Evidence that a complete stress release may have contributed to the record-breaking 2011 Tohoku earthquake. Both the sedimentary formations above and below the plate boundary fault are in the normal fault stress state, where vertical stress exceeds maximum horizontal stress. The new data are consistent with previous results above the fault, which is located at the boundary between the North American plate and the subducting Pacific plate, indicating that combining geophysical data and core samples to investigate stress states comprehensively is effective.
The great earthquake that caused the tsunami in northeastern Japan in 2011 is still remembered for its destructive power. The seismic nature of this calamity, also known as the Mw 9.0 Tohoku earthquake, was not initially clear. While earthquakes caused by built-up tectonic stress in reverse faulting had only been partially released. In previous studies where complete releases were proposed, the hypothesis was based on seismicity observation and simulation, or on direct stress measurement data above the fault only using log data.
Knowledge about stress changes before and after this earthquake, both above and below a gently dipping fault, can provide us with insights into how fault slipping caused the ensuing tsunami.
Weiren Lin
A team of researchers from Kyoto University has discovered evidence that a complete stress release may have contributed to the record-breaking event.
“The minor differences between maximum and minimum post-earthquake horizontal stresses near the fault suggest that the Tohoku earthquake occurred upon a complete stress release,” explains lead author Weiren Lin.
The team found that both sedimentary formations above and below the plate boundary fault lie in the stress state of normal faults in which vertical stress is greater than maximum horizontal stress.
“Knowledge about stress changes before and after this earthquake, both above and below a gently dipping fault, can provide us with insights into how fault slipping caused the ensuing tsunami,” reflects the author.
Lin’s team was able to collect data on the stress state above the Tohoku earthquake’s source fault, which is located at the boundary between the North American plate and the subducting Pacific plate. Geophysical data for the stress state below this zone, on the other hand, was unreliable.
To address this issue, the team examined one of four drill core samples collected from below the source fault by the Japan Trench Fast or JFAST – Drilling Project and was the first to successfully reveal the stress state at that depth.
“Our new data show good consistency with previous results above the fault, suggesting that combining geophysical data and core samples to comprehensively investigate stress states is effective.”
